This invention relates to an apparatus and method, generally for optical milling/shaping/reprofiling of solid organic materials, and specifically for reshaping the anterior surface of the cornea of the human eye for the purpose of correcting refractive defects.
Recent investigations have demonstrated the property of high energy photon radiation in the ultraviolet, such as is generated by the argon fluoride excimer laser (AFEL), to break the intermolecular bonds of organic materials--in effect disintegrating/ablating molecules exposed to this radiation while leaving the adjacent unexposed molecular structure intact resulting in an optically smooth interface. Because existing precision molding and optical lathing techniques can rapidly and accurately produce plastic optics, production of these using the AFEL does not, as yet, offer a superior means of manufacture. However, application of AFEL to living corneal tissue does offer a means for optically accurate corneal surface reshaping not achievable by other means, and with minimal diminution in transparency and trauma to adjacent interface cells. This characteristic has led to the application of the AFEL to corneal surgery including the refractive altering procedures of radial keratotomy, keratomileusis (corneal keratectomy), and anterior corneal surface modification (photorefractive keratectomy), the latter having greater potential for achieving optical accuracy and predictability than the preceding methods.
Existing methods being applied in photorefractive keratectomy involve the use of the scanning slit-beam lasers and constricting diaphragm controlled beams such as are described in U.S. Pat. Nos. 4,729,372, 4,770,172, 4,903,695, 4,941,093, 4,732,148. These inventions all use an en face method, that is, they direct a collimated laser beam collinear with the axis of the eye head-on onto the cornea. Concern over such direct radiation entering the eye is addressed in U.S. Pat. No. 4,840,175 by first removing a corneal slice (lamellar keratectomy) and ablating this slice in vitro. Another means for avoiding direct radiation into the eye is found in U.S. Pat. No. 4,724,522 by Belgorod which proposes a tangentially directed laser beam formed by mirrors mechanically rotating about the axis of the cornea to lathe the corneal surface; such a one or two dimensional technique is hampered by its extreme aiming accuracy requirements and being very time consuming in its application.
It is an object of the present invention to reduce the possible trauma associated with en face corneal ablation by use of a tangential means which avoids the problems of being highly sensitive and time consuming inherent in rotating-optical and/or small-spot-scanning excimer laser delivery systems.
A three dimensional tangential technique is presented in the European Patent Application 0222537 of York that avoids the two above-cited problems of the Belgorod patent, but does so at the expense of being limited by having a mirror element capable of generating only single fixed corneal shape, and further suffers from an excessive variation of radiation intensity (fluence) upon the corneal surface.
It is a further object of this invention to provide a method and apparatus incorporating a three dimensional tangential approach that allows the generation of a continuum of surface curvatures using a single optical device while at the same time providing a uniform radiation intensity over the full surface to be ablated.
Resorting to a tangential technique strictly because of a concern AFEL radiation could enter the eye and do damage to the inner eye, is unfounded as can be demonstrated by application of Beer's law, which states that light transmission varies as the power of ten raised to the negative product of the absorbance and the depth of penetration. At a wavelength of 193 nanometers (nm), 90% of the energy is absorbed in the first 10 microns (um) of corneal surface depth, and at the posterior surface of a cornea of normal thickness, approximately ten to the minus 60th power of the surface energy exists. Furthermore, much photo-ablative refractive surgery has already been performed without revealing any such damage. However, this fact alone does not imply that there are no differences between en face and tangential methods; the high energy AFEL pulses give rise to phonon shock waves, which if severe enough can damage the endothelium of the cornea or other eye structures. It can be reasoned that such undesired effects would be reduced by the glancing action of tangential radiation compared to the direct impact of en face radiation. Of even greater significance is the property of tangential radiation to cease ablating beyond a certain point which contrasts with en face radiation--i.e., the extent of tangential ablation for a given radiation intensity is distance dependent and hence self limiting, whereas the en face method is time dependent only. Essentially, it is this difference from en face methods that the present invention uses to enable the ablation of a distorted anterior cornea surface, having either regular or irregular astigmatism, to produce a corneal surface matching the smoothness and uniformity of that of the emmetropic human eye.
The cited patents of York and Belgorod both imply that the final shape imparted to the cornea matches that of the three dimensional locus of points tangential to a surface of desired curvature, referred to as a caustic. In truth, the actual surface ultimately imparted to the cornea is defined not by this caustic, but by a surface of shallower curvature, displaced from the caustic, and defined as the three dimensional locus of points where the rays intersect this new curvature at a constant, non-zero angle. The value of this angle, called a grazing angle, corresponds to the angle where the absorption of radiation energy into the cornea or ablatable material just rises above the ablation threshold of the particular material, the remainder of radiation being reflected away from the surface. Because the value of the grazing angle and radiation intensity are inversely related, ablation depth varies with intensity; thus to insure that the ablated surface curvature differs from the caustic curvature by a constant value, radiation intensity should be maintained substantially uniform.
That the York patent results in intensity variations amounting to orders of magnitude across the cornea surface area typically ablated, can be qualitatively demonstrated as follows: Beginning with York's collimated laser beam having an annular cross section of uniform intensity, this annular beam is intercepted by an annular mirror and is reflected in such a way as to form a dome of intersecting rays (i.e. a caustic) that impinge upon the cornea. Next, two equal width concentric bands on the mirror are considered: The first is a band around the periphery of the mirror, the second is a band around the center hole of the mirror. Energy reflected within the first band impinges on the cornea in the shape of a circle centered on the axis of the cornea having a radius r. Energy from the second band is reflected to the periphery of the cornea at a distance R from the axis, producing a band of approximate width r. Then, since the energy E is approximately equal in both bands, the intensity of radiation due to the first band is I.sub.1 =E/.pi.r.sup.2, while the second band causes an energy intensity on the cornea of I.sub.2 =E/2.pi.Rr, giving the ratio I.sub.1 /I.sub.2 =2R/r. Although these formulas are approximate, they illustrate the fact that York's method causes the radiation intensity upon the cornea to vary inversely with the radial distance from the axis. For example, the variation between the intensity within a 0.25 mm radius around the corneal axis is higher than the intensity at a peripheral distance of 4 mm by more than a factor of 30. Further, the fact that York claims only a single reflecting precludes any means for eliminating the inherent non-uniformity because the desired shape of the cornea uniquely dictates the shape of the mirror leaving no additional degree of freedom to impose a constraint to simultaneously achieve uniform intensity.
Further, in York's method, to insure that radiation intensity rises above the threshold value of ablation in the peripheral region of the cornea, the radiation intensity near the axis of the cornea must rise to values in excess of 1 joule/cm.sup.2. Such high intensities have been associated with thermal effects on corneal tissue, and consequently can result in damage to adjacent tissue, thus defeating the intended effect of excimer laser photoablation. (See for example, Aron-Rosa, et.al. "Keratorefractive Surgery with the Excimer Laser" Am. Journal Opht. Nov. 1985, pp 741-742.)
Therefore, it is a principal object of this invention to reduce or eliminate intensity variations, such as occur in the York patent, so as to prevent thermal damage and insure that the full corneal surface is subjected to substantially uniform intensity radiation of an optimum value.
Inherent in the patent of York, is the necessity that a significant portion of the laser beam be masked off to produce the required annular beam cross section. Because the caustic surfaces needed for corneal ablation are on the order of one square centimeter or more, present excimer laser technology is limited to about the radiation intensity mandated for such an area, and therefore any inefficiencies can be detrimental.
So, it is also an object of this invention to utilize the available laser beam with maximum efficiency.
An essential optical element of this invention is a specialized conical lens called an axicon which in conjunction with a specialized mirror forms a catadioptric system which achieves the object of the invention relating to uniformity of radiation intensity. In the patent of Marshall, et.al. (U.S. Pat. No. 4,941,093) use is made of axicon lenses to produce continuously variable size annular beams for en face photoablation. However, the intensity within the annulus is not uniform making the successful application of an axicon or combination of axicons for photoablation, somewhat dubious. In the present invention, the application of an axicon is totally difference from the Marshall patent; this is not only because the tangential method rather than the en face method is being used here, but much more importantly, it is because the intensity non-uniformity of the axicon, in combination with the complementary intensity non-uniformity of the mirror of the present invention, creates the sought-after caustic of uniform intensity.
In addition to the refractive errors of myopia, hyperopia, and astigmatism, the inability of the eye to change its focus or accommodate, known as presbyopia, is an unavoidable problem that afflicts all humans past middle age. Recent work in the field of refractive aspheric and diffraction contact or interocular lenses has shown promise in the amelioration or correction of this visual problem.
It is a further object of this invention to provide a method and apparatus that will enable corneas to be ablated throughout a continuum of aspherical curvatures for the purpose of optimizing visual acuity over as large a depth of field as possible without changing the accommodation of the lens of the eye.